Electric motors and commutators therefor

Abstract

 An electric motor (1, Fig. 1) has a commutator (24, Figs. 2 and 3) which has commutator bars (42) having two upstanding tangs (44, 46) for the attachment of the armature coil wires (56). Furthermore the wires (56) may be wound as a pair of wires (70, 72, Fig. 5) one wire (70) being attached to one tang (44) and the other wire (72) being attached to the other tang (46).

Description

[0001] This invention relates to electric motors, particularly dc motors and to commutators for such motors.

[0002] Electric motors have an armature comprising a usually laminated core or stack on the spokes of which are wound coils of wire. The ends of the wires are attached to a commutator on the armature and through which the coils are supplied with electric current to induce an armature magnetic field. The current is supplied by brushes bearing against the commutator. The armature is positioned in a stator magnetic field, usually provided by field coils (in the case of ac motors) or by permanent magnets (in the case of dc motors). The interaction of the magnetic fields of the stator and armature causes the armature to rotate in the stator.

[0003] The size of electric motors is determined by the required power output. Large power requirements usually dictate ac motors because high dc voltages and current capacities are not generally available. Hitherto this limitation has extended down to motors used in for example hand operated power tools, but new developments in battery technology are making sufficiently high dc electrical power available to make dc motors attractive in such tools. With the batteries now developed which are small and rechargeable, they can be incorporated in the tool or a separate battery pack.

[0004] Two factors have hitherto kept cordless tools at the low power end of the spectrum. Firstly, the batteries which might be suitable for power tools have not had sufficient capacity to provide high electrical power for worthwhile periods of time. Secondly, it is problematic to design dc motors of small size for use in power tools and operating at low voltage to provide high power outputs.

[0005] The high voltage of ac motors means that they can be relatively small compared with dc motors of corresponding power output. The reasons for this are somewhat complicated.

[0006] Given a dc motor of a particular size, and operated at a particular voltage, the power output is primarily a function of the current through its armature coils, given the stator magnetic field. The speed of such a motor is, under no-load conditions, determined by the number of turns in the armature coils. The fewer the turns, then the higher the speed of the motor, because the back emf induced, which slows the motor and balances against the driving force, is directly related to the number turns in the armature coils. Since the speed is also usually a prerequisite factor, so then the number of turns in the coils are also predetermined. Thus the overriding factor controlling the power output is simply the thickness of the armature coil wires which can be accommodated within the size constraints of the motor. The thickness of the wire of course determines its resistance and thus the current it passes at the given voltage.

[0007] The problem, which it is an object of the present invention to overcome, is that the thickness of the wire which can be incorporated is such as to make winding of the coil in presently available winding machines somewhat difficult. The coil windings are connected to the commutator.

[0008] The commutator comprises an electrically insulating material core mounted on the armature shaft and on which are set commutator elements or bars against which the brushes bear. The commutator may take several forms, for example, it may be cylindrical with the commutator elements being disposed around its periphery and the brushes then bearing on them in a radial direction. On the other hand it may be disc like where the commutator elements are set in the core along radial axes of the armature shaft and in which event the brushes act axially against the commutator.

[0009] The commutator is secured on the armature adjacent the armature stack on which the coils are wound. In the case of cylindrical commutators, that end of each commutator element which lies nearer the stack is provided with an upstanding tang. The tang is bent backwards so that the coil wire may be wound around it and it is subsequently bent further back and stake welded to the wire and commutator element surface to securely fix the wire to the commutator.

[0010] In the size of motors with which the present invention is concerned, the thickness of the coil wire approaches the same dimensions as the commutator element tangs, and consequently the latter are barely strong enough to withstand the rigours associated with the mechanical winding process. This is because the tangs are formed from material which is integral with the commutator elements and which themselves are formed from a tube or from sheet material folded into a tube. Thus the tangs can only be as thick as the elements, and in any event cannot be so thick that, in the bending back stage of their formation, they are likely to fracture. Besides which, it is an expensive solution to thicken the tangs if this also thickens the elements, since the commutator elements do not require such thickness for their own satisfactory operation.

[0011] Furthermore, where a given commutator element connects to two armature coils, the tang on it usually catches just one piece of wire since the same wire is used to wind more than one coil. The tang may however bend and break under the stress of the wire being wound around it. However, it is more likely to break when the motor is in use, particularly in arduous environments, through the stresses imposed on it by the coil wires. It should be appreciated that each tang is effectively acted upon by two wires from two different directions. A particular problem occurs where the two wires are not, in fact, one single wire wound around the tang, but are the ends of two separate wires and so, here, the tang must grip two wires as opposed to just one. With the wires being of the same order of thickness of the tang, this tang poses particular problems of potential breakage. As mentioned above, the wires are fixed to the tangs after winding by bending the tangs over the wires and stake welding them to the wires and to the commutator bar beyond the coil wire and so as to close the loop formed by the tang around the wire. Again, two wires make this difficult to achieve, particularly when the wire is thick. Thus as mentioned above, it is an object of the present invention to overcome these problems, or at least to mitigate their effects.

[0012] In accordance with the present invention, therefore, there is provided an electric motor comprising an armature stack on which is wound the wires of a plurality of armature coils and a commutator secured on said armature, which commutator comprises a core around which is disposed a plurality of commutator elements to which said wires are connected, each commutator element having at least two upstanding tangs about at least two of which said wires are connected. Preferably there are two tangs on each element.

[0013] By this simple expedient the stress on each individual tang can be reduced. Firstly, during winding, two tangs take the strain imposed by the winding mechanism. Secondly, after the tangs have been bent over and welded, each tang effectively is only acted upon by one wire, because the wire between two tangs would not exert any strain on either tang but, if anything, would rather support and balance the stresses on them. Thirdly, no tang needs to secure two wires, because the two ends of the coil wire could be secured one to each in the pair of tangs on that element.

[0014] The invention also provides a commutator for such motor comprising a core of electrically insulating material around which is set a plurality of commutator elements against which motor brushes are adapted to bear when in use, at one end of each commutator element there being formed at least two upstanding tangs for attachment thereto of armature coil wires. Preferably the core is annular and cylindrical and the commutator elements are set around its periphery.

[0015] Apart from the limitation imposed by the tangs to the thickness of the coil wire which is possible, (a limitation which is in any event largely eliminated by the present invention), there is a further limitation to the maximum thickness of wire achievable. This is the physical size of the space available between the spokes of the armature stack to receive the predetermined number of turns of wire. The design of the laminated armature stack maximises the space available; but however large this is, it imposes a limit on the thickness of the coil wires which can pass through it. In any event, while two tangs may provide sufficient strength it does nothing to alleviate the difficulty of handling relatively thick wire.

[0016] Thus it has been suggested to wind each coil from a set of at least two wires lying in parallel with respect to one another.

[0017] An immediate benefit from this is that winding two wires of 1/

times the thickness of a single wire provides identical current carrying capacity, but places far less strain, not just on the winding mechanism, but also, especially, on the tangs of the commutator. Secondly, although, theoretically, it is not possible to increase the total cross sectional area of round wires passing through a given cross-section simply by using more wires of thinner diameter (indeed in theory the total cross-sectional area achievable is the same, regardless of the number and diameter of the individual wires), in practice, given the nature of motor armature geometry, it is possible to achieve greater cross-sectional area by using multiple thinner wires in parallel rather than single thicker wires. Nevertheless it should be emphasised that the primary benefit from using multiple parallel wire coils is realised in the practicality of the armature so wound and not because of the increased power output achievable.

[0018] Preferably, therefore, each coil comprises a set of at least two wires lying in parallel with respect to one another, each of said wires in each set being connected to a different tang on the same commutator element. Preferably there are two wires in each set and two tangs on each commutator element.

[0019] Thus it is apparent that the primary benefit achieved by the present invention is in this latter respect, in that, not only does two tangs on each commutator bar provide increased strength of connection for the coil wires, and not only does the division of the armature coils into two wires in parallel make winding easier and more efficient and in practice actually allow greater overall thickness to be achieved, but also the combination of these two features means that each tang is actually being wound to a thinner wire and consequently the strength characteristics of the final product are further enhanced despite the greater power output achievable by the motor.

[0020] The invention is further described hereinafter, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a side section through a motor according to the present invention;

Figure 2 is a section through a commutator according to the present invention;

Figure 3 is a section on the line III-III in Figure 1;

Figure 4 is a schematic diagram of the armature coil layout of a motor according to the present invention;

Figure 5 is as Figure 4 but in relation to a preferred embodiment of the present invention;

Figure 6 is a section through a partly wound armature stack along the line VI-VI in Figure 1; and

Figure 7 is a side view, partly in section of a wound armature.

[0021] In the drawings, a dc electric motor 1 has a cylindrical housing 10 having end caps 12, 14 rotatably journalling an armature shaft 16 through bearings 18, 20 formed in the end caps 12, 14 respectively.

[0022] The armature shaft carries an armature core or stack and coils 22 and a commutator 24. The end cap 12 mounts brushes 26 adapted to bear against the commutator 24 and by means of which electrical current is passed to the armature coils 22. The housing 10 contains permanent magnets 28 providing the stator field for the motor. The armature shaft 16 also carries an impeller 30 for cooling the armature coils as the motor operates. The shaft 16 is splined at its output end 32 for driving a variety of power tools or such like.

[0023] Power tools, particularly hand held tools, are ideally light and powerful, and preferably not limited by a requirement for connection to a fixed power source. Batteries are now becoming available which are relatively light and yet capable of providing sustained high power dc electrical output.

[0024] Although batteries can be carried in a separate power pack, this does offer limitations to the uses of the tool. Preferably the batteries are carried in the tool itself and are rechargeable there, or at least replaceable with fresh, fully charged batteries from time to time.

[0025] In any event, it is desirable that the other main weight component of the tool, that is to say, its electrical motor, be both light and powerful.

[0026] The commutator 24, as seen in Figures 2 and 3, comprises an annular core 40 of plastics or preferably phenolic material or such other electrically insulating material as may be suitable. On the cylindrical surface of the core 40 are set arcuate commutator elements or bars 42 and, in the case illustrated herein, there are five of them. Each bar 42 is provided at one end 48 thereof with two upstanding tangs 44, 46. The tangs 44, 46 are adapted to catch the coil wires as they are wound on the armature core.

[0027] The armature core comprises a series of ferromagnetic discs 50 (see Figures 6 and 7) collected together in a stack and carried on the armature shaft 16 on an aperture 51 formed in each disc. Each disc has five spokes 52 which between them define five coil cavities or slots 54. These slots are filled with armature coil wires 56 (as described further below) only some of which are shown in Figure 6.

[0028] Figure 4 shows schematically the arrangement of the coil wiring 56. As can be seen, one long wire is used to form five coils A to E around the spokes 52 and in the slots 54 between them. Thus each slot 54 carries one leg of two coils and each coil bridges two spokes.

[0029] The coil wiring starts at one tang 44 on one commutator bar, say bar 42c. After winding around the spoke pair 52b, c a number of turns, the wire is hooked around the tangs 44, 46 on bar 42b and winding continues around spokes 52a, b. This process continues until all the coils A to E are wound when the wire is then hooked and terminated on tang 46 back on bar 42c.

[0030] The tangs 44, 46 are then stake welded to securely connect the wiring to the commutator bars 42 and not least to break the insulative coating on the wires 56 and make electrical connection between the coils 56 and commutator bars 42.

[0031] The brushes supplying the commutator and armature windings with current are disposed diametrically opposite one another as shown in Figure 1 and act in a radial direction. As the commutator rotates, two different situations arise. Referring to Figure 4, firstly one brush abuts fully against, say bar 42b, while the other brush electrically bridges bars 42d, e. In this event current is supplied in parallel to coils B and A in series and to coils C and D in series. The coils are arranged so that the magnetic fields generated by the current in these coils is in the same direction and at an angle to the magnetic field of the stator so that the armature rotates. On rotation the brushes move to the second situation in which one brush fully abuts bar 42b and the other abuts fully with 42e (depending on the direction of current flow). In this event, current is supplied in parallel to coils B and A in series (as before) and to coils C, D and E in series.

[0032] Thus as the armature rotates, current passes through different numbers of coils but the arrangement is such that the average resistance to current of the coils is in fact about 1.1 times the resistance of one coil. As mentioned above, the speed of the motor under no-load conditions is determined, other things being equal, by the number of turns in each coil. Given the dimensions of the slots 54 there is a limit to how thick the wires 56 can be made, particularly where they exit the slots 54 at either end and overlap with each other.

[0033] Nevertheless even with the maximum thickness achievable the provision of a pair of tangs 44, 46 on each bar mitigates the stresses and strains they experience, not just in the winding process but also in use when the motor has been incorporated in some power tool or such like.

[0034] However, as illustrated schematically in Figure 5, the coil wiring 56 may be divided between two wires 70, 72 disposed in parallel with respect to one another, each of a diameter perhaps somewhat larger than 1/

times the diameter of a single wire. If room permits the double wires can be even larger, but in any event two wires are easier to wind than one thick wire and would place less strain individually and collectively on the winding apparatus and, perhaps equally importantly on the tangs of the commutator.

[0035] Here it is to be appreciated that each wire 70, 72 attaches to one each of the tangs 44, 46 so that the size balance between tang and wire is returned to more usual proportions. One tang which begins and ends each winding must have two wires under it, of course, but as stated above, this can be accepted with the size balance being reverted to favour the tang.

[0036] It is a matter of choice whether the wires 70, 72 are wound simultaneously or first one and then the other. The latter is simpler from the winding mechanism's point of view but in fact presents more difficulties from bulkiness in the overlap regions of the coils at the exits to the slots 54. Thus the former method of winding is preferred.

Claims

1. An electric motor comprising an armature stack on which is wound the wires of a plurality of armature coils and a commutator secured on said armature, which commutator comprises a core around which is disposed a plurality of commutator elements to which said wires are connected, each commutator element having at least two upstanding tangs about at least two of which said wires are connected.

2. A motor as claimed in Claim 1 in which each of said coils comprises a single wire connected to the tangs on each commutator element.

3. A motor as claimed in Claim 1 in which each of said coils comprises a set of at least two wires lying in parallel with respect to one another, each of said wires in each set being connected to a different tang on the same commutator element.

4. A motor as claimed in Claim 3 in which there are two wires in each set.

5. A motor as claimed in any preceding claim in which there are two tangs on each commutator element.

6. A motor as claimed in any preceding claim in which the core of said commutator is annular and substantially cylindrical and said elements are disposed around the periphery thereof.

7. A motor as claimed in any preceding claim, which is a five coil motor, each coil being wound around spokes of an armature core having five of said spokes and the commutator having five commutator elements.

8. A motor as claimed in Claim 7 when appendant to Claim 2, in which the coils are wound as schematically shown in Figure 4 of the accompanying drawings.

9. A motor as claimed in Claim 7 when appendant to Claims 4 and 5, in which the coils are wound as schematically shown in Figure 5 of the accompanying drawings.

10. A motor as claimed in Claim 3 or in any of Claims 4, 5, 6, 7 or 9 when appendant to Claim 3, in which all wires in said set are wound on the coils simultaneously.

11. A commutator for an electric motor comprising a core of electrically insulating material around which is set a plurality of commutator elements against which motor brushes are adapted to bear when in use, at one end of each commutator element there being formed at least two upstanding tangs for attachment thereto of armature coil wires.

12. A commutator as claimed in Claim 11, in which said core is annular and substantially cylindrical and said elements are disposed around the periphery thereof.

13. A commutator as claimed in Claim 11 or 12 in which there are two tangs on each element.

14. An electric motor substantially as hereinbefore described with reference to Figures 1 and 4 to 7 of the accompanying drawings.

15. A commutator for an electric motor substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

Drawing